The heat range of the plug is an indicator of how quickly the plug cools off between firings. A colder heat range will cool off quicker than a hotter range.

You want to run as hot a plug as the engine can stand, this ensures that deposits get burnt off so the engine will run well for a long time. To hot a plug and it can cause detonation and the tips will erode away faster.

The stock heat range is just fine, one heat range cooler will not make a difference if the engine was running well with the stock range.

The 3923 mentioned above is one heat range cooler, a 3924 is stock. These plugs have a 5/8" hex for the socket (but are the same other wise) instead of the 13/16" of the 64 or 65. The 5/8" hex makes it easier getting the plug past header tubes, the 3924 plug is what comes stock in 3.9/5.2/5.9 liter magnum engines.

I typically use Autolites simply because the local Walmart has them and it saves me a few miles of driving to a parts store to get something else. I would avoid the gimik plugs like E3 and Splitfires, once the voltage potential builds up enough to jump a spark you only get one regardless of how many or the configuration of the ground electrode.

The platinum and an iridium plugs last longer than a conventional plug and that is why they exist, not to make more power or increase mileage. They were developed so the manufacturers could extend plug changes to 100,000 miles.